Complexes with tridentate ligands

ABSTRACT

The present invention relates to organic light emitting devices (OLEDs), and more specifically to phosphorescent organic materials used in such devices. More specifically, the present invention relates to emissive phosphorescent material which comprise at least one tridentate ligand bound to a metal center, wherein at least one of the bonds to the tridentate ligand is a carbon-metal bond.

This application is a continuation-in-part of U.S. application Ser. No.10/880,384, filed Jun. 28, 2004, which is a continuation-in-part of U.S.application Ser. No. 10/849,301, filed May 18, 2004, each of which areincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs),and more specifically to phosphorescent organic materials used in suchdevices. More specifically, the present invention relates to emissivephosphorescent material which comprise at least one tridentate ligandbound to a metal center, wherein at least one of the bonds to thetridentate ligand is a carbon-metal bond.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in organic opto-electronic devices. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may be comprised of athick and reflective metal layer having a high electrical conductivity.Similarly, for a device intended to emit light only through the topelectrode, the bottom electrode may be opaque and/or reflective. Wherean electrode does not need to be transparent, using a thicker layer mayprovide better conductivity, and using a reflective electrode mayincrease the amount of light emitted through the other electrode, byreflecting light back towards the transparent electrode. Fullytransparent devices may also be fabricated, where both electrodes aretransparent. Side emitting OLEDs may also be fabricated, and one or bothelectrodes may be opaque or reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

The carbene ligand has been well known in organometallic chemistry, andis used to generate a wide range of thermally stable catalyticmaterials. The carbene ligands have been employed both as active groups,directly engaged in the catalytic reactions, and serving a role ofstabilizing the metal in a particular oxidation state or coordinationgeometry. However, applications of carbene ligands are not well known inphotochemistry.

One issue with many of the existing organic electroluminescent compoundsis that they are not sufficiently stable for use in commercial devices.This has been particularly true of phosphorescent emissive materialsthat emit in the blue portion of the spectra. An object of the inventionis to provide a class of organic emissive compounds having improvedstability. An object of the invention is to provide a class of organicemissive compounds that can emit light with various spectra, includinghigh energy spectra such as blue, in a stable manner.

SUMMARY OF THE INVENTION

The present invention is directed to an organic light emitting devicehaving an emissive layer comprising a phosphorescent organometallicemissive material having at least one tridentate ligand bound to a metalcenter through at least one carbene-metal bond. In a preferredembodiment, the emissive material comprises two tridentate ligands,which may be the same or different, bound to the metal center, whereinat least one of the tridentate ligands has a carbene-metal bond.

In a preferred embodiment, the invention is directed to an organic lightemitting device comprising an anode, a cathode, and a phosphorescentemissive region disposed between the anode and the cathode, wherein theemissive region comprises an emissive material having the formula I:

wherein:

-   M is a second or third row transition metal; the dotted lines    represent optional double bonds;-   L is an ancillary ligand;-   X¹ is selected from C and N;-   X² is selected from C, N, O, S and P;-   X³ is selected from C and N; each Q is independently selected from a    chemical bond and —C(R′)₂—;-   Y¹ is C or N;-   Y² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N;-   R³ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   a is 1 or 2;-   R³ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   c is 1 or 2;-   alternatively, R¹ and R³, taken together with Y¹ and Y², form a 5-    or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   Z¹ is C or N;-   Z² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N;-   R is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   b is 1 or 2;-   R⁴ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   d is 1 or 2;-   alternatively, R² and R⁴, taken together with Z¹ and Z², form a 5-    or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   R⁵ and R⁶ are independently selected from H, alkyl, alkenyl,    alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl and heteroaryl; or    alternatively, R⁵ and R⁶, taken together with X¹, X² and X³ form a    5- or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   additionally or alternatively R¹ and R⁵ taken together with X¹, Q    and Y′ form a 5- or 6-membered cyclic group or a 8- to 10-membered    fused bicyclic group, which may be optionally substituted with one    or more substituents independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   additionally or alternatively R² and R⁶ taken together with X³, Q    and Z′ form a 5- or 6-membered cyclic group or a 8- to 10-membered    fused bicyclic group, which may be optionally substituted with one    or more substituents independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   each R′ is independently selected from H, alkyl, alkenyl, alkynyl,    aralkyl, aryl and heteroaryl;-   n is 1 or 2; and-   m is 0 to 3, wherein when n is 1, m is 1 to 3, and when n is 2, m is    0;-   wherein at least one of X², Y² or Z² is C and is selected to be a    carbene donor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows the emission spectra for compound Os1.

FIG. 4 shows the emission spectra for compound Os2.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that anorganic material that exhibits phosphorescence at liquid nitrogentemperatures typically does not exhibit phosphorescence at roomtemperature. But, as demonstrated by Baldo, this problem may beaddressed by selecting phosphorescent compounds that do phosphoresce atroom temperature. Representative emissive layers include doped orun-doped phosphorescent organometallic materials such as disclosed inU.S. Pat. Nos. 6,303,238 and 6,310,360; U.S. Patent ApplicationPublication Nos. 2002-0034656; 2002-0182441; 2003-0072964; andWO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN or CO.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference in their entireties. Anode 115 may be opaque and/orreflective. A reflective anode 115 may be preferred for sometop-emitting devices, to increase the amount of light emitted from thetop of the device. The material and thickness of anode 115 may be chosento obtain desired conductive and optical properties. Where anode 115 istransparent, there may be a range of thickness for a particular materialthat is thick enough to provide the desired conductivity, yet thinenough to provide the desired degree of transparency. Other anodematerials and structures may be used.

Hole transport layer 125 may include a material capable of transportingholes. Hole transport layer 130 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of 50:1,as disclosed in U.S. Patent Application Publication No. 2002-0071963 A1to Forrest et al., which is incorporated by reference in its entirety.Other hole transport layers may be used.

Emissive layer 135 may include an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160.Preferably, emissive layer 135 contains a phosphorescent emissivematerial, although fluorescent emissive materials may also be used.Phosphorescent materials are preferred because of the higher luminescentefficiencies associated with such materials. Emissive layer 135 may alsocomprise a host material which may be capable of transporting electronsand/or holes, doped with an emissive material that may trap electrons,holes, and/or excitons, such that excitons relax from the emissivematerial via a photoemissive mechanism. Emissive layer 135 may comprisea single material that combines transport and emissive properties.Whether the emissive material is a dopant or a major constituent,emissive layer 135 may comprise other materials, such as dopants thattune the emission of the emissive material. Emissive layer 135 mayinclude a plurality of emissive materials capable of, in combination,emitting a desired spectrum of light. Examples of phosphorescentemissive materials include Ir(Ppy)₃. Examples of fluorescent emissivematerials include DCM and DMQA. Examples of host materials include Alq₃,CBP and mCP. Examples of emissive and host materials are disclosed inU.S. Pat. No. 6,303,238 to Thompson et al., and U.S. ApplicationPublication No. US-2004-0209116 A1, each of which is incorporated byreference in its entirety. Emissive material may be included in emissivelayer 135 in a number of ways. For example, an emissive small moleculemay be incorporated into a polymer. This may be accomplished by severalways: by doping the small molecule into the polymer either as a separateand distinct molecular species; or by incorporating the small moleculeinto the backbone of the polymer, so as to form a co-polymer; or bybonding the small molecule as a pendant group on the polymer. Otheremissive layer materials and structures may be used. For example, asmall molecule emissive material may be present as the core of adendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the photoactive properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsinvolved in light emission. A ligand that is photoactive in one moleculemay be ancillary in another. These definitions of photoactive andancillary are intended as non-limiting theories.

Electron transport layer 145 may include a material capable oftransporting electrons. Electron transport layer 145 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in U.S. Patent Application Publication No.2002-0071963 A1 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers may be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) energy levelof the electron transport layer. The “charge carrying component” is thematerial responsible for the LUMO energy level that actually transportselectrons. This component may be the base material, or it may be adopant. The LUMO energy level of an organic material may be generallycharacterized by the electron affinity of that material and the relativeelectron injection efficiency of a cathode may be generallycharacterized in terms of the work function of the cathode material.This means that the preferred properties of an electron transport layerand the adjacent cathode may be specified in terms of the electronaffinity of the charge carrying component of the ETL and the workfunction of the cathode material. In particular, so as to achieve highelectron injection efficiency, the work function of the cathode materialis preferably not greater than the electron affinity of the chargecarrying component of the electron transport layer by more than about0.75 eV, more preferably, by not more than about 0.5 eV. Similarconsiderations apply to any layer into which electrons are beinginjected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2 and 6,576,134 B2, which are incorporated by reference intheir entireties, disclose examples of cathodes including compoundcathodes having a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thepart of cathode 160 that is in contact with the underlying organiclayer, whether it is a single layer cathode 160, the thin metal layer162 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 130 may be disposed between emissive layer 135and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 145.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and U.S. Patent ApplicationPublication No. 2002-0071963 A1 to Forrest et al., which areincorporated by reference in their entireties.

As used herein, and as would be understood by one skilled in the art,the term “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode oran organic layer, into an adjacent organic layer. Injection layers mayalso perform a charge transport function. In device 100, hole injectionlayer 120 may be any layer that improves the injection of holes fromanode 115 into hole transport layer 125. CuPc is an example of amaterial that may be used as a hole injection layer from an ITO anode115, and other anodes. In device 100, electron injection layer 150 maybe any layer that improves the injection of electrons into electrontransport layer 145. LiF/Al is an example of a material that may be usedas an electron injection layer into an electron transport layer from anadjacent layer. Other materials or combinations of materials may be usedfor injection layers. Depending upon the configuration of a particulardevice, injection layers may be disposed at locations different thanthose shown in device 100. More examples of injection layers areprovided in U.S. patent application Ser. No. 09/931,948 to Lu et al.,which is incorporated by reference in its entirety. A hole injectionlayer may comprise a solution deposited material, such as a spin-coatedpolymer, e.g., PEDOT:PSS, or it may be a vapor deposited small moleculematerial, e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO energy level that actuallytransports holes. This component may be the base material of the HIL, orit may be a dopant. Using a doped HIL allows the dopant to be selectedfor its electrical properties, and the host to be selected formorphological properties such as wetting, flexibility, toughness, etc.Preferred properties for the HIL material are such that holes can beefficiently injected from the anode into the HIL material. Inparticular, the charge carrying component of the HIL preferably has anIP not more than about 0.7 eV greater that the IP of the anode material.More preferably, the charge carrying component has an IP not more thanabout 0.5 eV greater than the anode material. Similar considerationsapply to any layer into which holes are being injected. HIL materialsare further distinguished from conventional hole transporting materialsthat are typically used in the hole transporting layer of an OLED inthat such HIL materials may have a hole conductivity that issubstantially less than the hole conductivity of conventional holetransporting materials. The thickness of the HIL of the presentinvention may be thick enough to help planarize or wet the surface ofthe anode layer. For example, an HIL thickness of as little as 10 nm maybe acceptable for a very smooth anode surface. However, since anodesurfaces tend to be very rough, a thickness for the HIL of up to 50 nmmay be desired in some cases.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. 09/931,948 to Lu et al., whichis incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,an cathode 215, an emissive layer 220, a hole transport layer 225, andan anode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVJD.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having two or moremono-, bi- and/or tridentate ligands, such that after the substituentsare added, one or more of the mono-, bi- and/or tridentate ligands arelinked together to form, for example, a tetradentate or hexadentateligand. Other such linkages may be formed. It is believed that this typeof linking may increase stability relative to a similar compound withoutlinking, due to what is generally understood in the art as a “chelatingeffect.”

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

The present invention is directed to a device having an emissive layercomprising a phosphorescent organometallic emissive material having atleast one tridentate ligand bound to a metal center through at least onecarbon-metal bond. The metal center is selected from the second andthird row transition metals and is preferably selected from Ru, Os, Re,Rh, Ir, Pd and Pt. In particularly preferred embodiments, M is selectedfrom Os and Ru, and in still more preferred embodiments, M is Os. Theemissive material may additionally contain ancillary ligand to fill thecoordination sphere of the metal center. In a preferred embodiment, theemissive material comprises two tridentate ligands, which may be thesame or different, bound to the metal center, wherein at least one ofthe tridentate ligands has a carbon-metal bond. In a particularlypreferred embodiment, the metal center is osmium. The device may containadditional emissive materials that may be phosphorescent emissivematerials or fluorescent emissive materials.

In a preferred embodiment of the invention, the carbon-metal bond to thetridentate ligand of the emissive material is a carbene-metal bond.Thus, in this embodiment, the emissive material comprises at least onetridentate ligand bound to a metal center through at least onecarbene-metal bond. The two other bonds to the metal center may beselected from (i) dative bonds from a heteroatom selected from N, O, S,and P, (ii) carbon-metal bonds that are not carbene-metal bonds, (iii)carbene-metal bonds, or any combination thereof.

As used herein, the term “carbene” refers to compounds having a divalentcarbon atom with only six electrons in its valence shell when notcoordinated to a metal. A useful exercise to determine whether a ligandincludes a carbene-metal bond is to mentally deconstruct the complex asa metal fragment and a ligand, and to then determine whether a carbonatom in the ligand that was previously bound to the metal is a neutraldivalent carbon atom in the deconstructed state. The resonance forms ofa preferred embodiment may be shown as:

This definition of carbene is not limited to metal-carbene complexessynthesized from carbenes, but is rather intended to address the orbitalstructure and electron distribution associated with the carbon atom thatis bound to the metal. The definition recognizes that the “carbene” maynot technically be divalent when bound to the metal, but it would bedivalent if it were detached from the metal. Although many suchcompounds are synthesized by first synthesizing a carbene and thenbinding it to a metal, the definition is intended to encompass compoundssynthesized by other methods that have a similar orbital structure andelectron configuration. Lowry & Richardson, Mechanism and Theory inOrganic Chemistry 256 (Harper & Row, 1976) defines “carbene” in a waythat is consistent with the way the term is used herein. Some referencesmay define “carbene” as a carbon ligand that forms a double bond to ametal. While this definition is not being used in the presentapplication, there may be some overlap between the two definitions. Avariety of representations are used to depict the bonding in suchcarbenes, including those in which a curved line is used to indicatepartial multiple bonding between the carbene carbon and the adjacentheteroatom(s).

Carbene ligands are especially desirable in OLED applications due to thehigh thermal stability exhibited by metal-carbene complexes. It isbelieved that the carbene, which behaves much as an electron donativegroup, generally bonds strongly to the metals, thus forming a morethermally stable complex than, for example, previous cyclometallatedcomplexes used as phosphorescent emitters.

Moreover, due to the nature of a carbene-metal bond, it is believed thatthe emissive molecule comprising a carbene-metal bond may have increasedstability, for example in comparison to a compound that is a non-carbeneanalog. It is further believed that the emissive spectra of the moleculeincluding a carbene-metal bond may be different from the emissivespectra of the analog without a carbene.

Metal-carbene complexes may be tuned to emit a wide variety of spectrafrom the near-ultraviolet across the entire visible spectra by theselection of substituents and/or chemical groups on the ligand(s). Moresignificantly, it may now be possible to obtain saturated blue coloremissions with peak wavelengths at about 450 nm. Because it is believedto be materially easier to reduce than to increase the triplet energy bytuning an emissive compound, the ability to make stable blue emitters atsuch high energies would also allow for the possibility of obtaining anycolor by reducing the energy so as to red-shift the emission.

The appropriate selection of substituents and/or chemical groupsattached to carbene ligands may also minimize quantum efficiency lossesassociated with increasing temperatures. The observable difference inlifetime measurements between emission at room temperature and at lowtemperatures (e.g. 77 K) is believed to be attributed to non-radiativequenching mechanisms that compete with phosphorescent emission. Suchquenching mechanisms are further believed to be thermally activated, andconsequently, at cooler temperatures of about 77 K, where energy lossdue to quenching is not an issue, quantum efficiency is about 100%. Itis believed that appropriate substituents on the carbene ligand, ordoping in a more rigid matrix, such as disclosed in Turro, “ModernMolecular Photochemistry”, University Science Books (1991), 109-10, mayincrease quantum efficiency at room temperature and correspondingly showlonger lifetimes.

In some embodiments, the triplet energy of the carbene complex has acorresponding wavelength in the deep blue or ultraviolet (UV) part ofthe spectra. In some embodiments, the phosphorescent emissive compoundhas triplet energy corresponding to a wavelength of less than 450 nm. Inpreferred embodiments, the triplet energy corresponds to a wavelength ofless than 440 nm, and in even more preferred embodiments, it correspondsto a wavelength less than 400 nm, which is believed to be in the UVregion of the spectrum, since 400 nm is believed to represent thecut-off between the UV and the visible regions of the spectrum. Suchhigh triplet energy may make these compounds useful in optically pumpingdown converting layers. For such applications, an overlap is preferredbetween the emission spectra of the ultraviolet carbene compound and theabsorption spectra of the down converting layer. It is believed thatwhen about 50% of the integral of the curve for the normalizedelectroluminescent spectra of the device is at a wavelength less thanabout 450 nm, there is sufficient energy to optically pump a downconverting layer. More preferably, greater than 90% of the emission maybe produced below 440 nm, as disclosed herein. Preferably, 50% of theintegral of the curve for the normalized electroluminescent spectra isless than about 440 nm, and more preferably, it is less than about 400mm. The wavelength cutoffs mentioned above are not intended to beabsolute limitations as they depend on the energy of the material to bepumped. It is also believed that these emissions may occur at roomtemperature.

The strong metal-carbon bond is also believed to lead to greaterspin-orbit coupling in metal carbene complexes. Moreover, the tripletenergy of coordinated carbenes are shown to be significantly higher thanpyridine analogs. The emission may be in the near-ultraviolet range ofthe spectrum even at room temperature. It is believed herein that metalcarbene complexes may be capable of emitting at similarly high energiesdue to the strong metal-ligand bond associated with carbene ligands.

The stability of metal-carbene complexes may also allow increasedversatility in the types of ligands and metals that may be used asphosphorescent emitters in OLEDs. The strong metal-carbene bond mayallow a variety of metals to form useful phosphorescent complexes withcarbene ligands to give novel emissive compounds.

The emissive materials of the present invention comprise a tridentateligand that is a photoactive ligand. A ligand is referred to as“photoactive” because it is believed that it contributes to thephotoactive properties of the emissive material. The emissive materialmay further include ancillary ligands. These ligands are referred to as“ancillary” because it is believed that they may modify the photoactiveproperties of the molecule, as opposed to directly contributing to thephotoactive properties. The definitions of photoactive and ancillary areintended as non-limiting theories. The ancillary ligands may be selectedfrom those disclosed in the following references:

U.S. Pat. Application Publ. No. 2002-0034656 (K&K 10020/15303), FIGS.11-50, U.S. Pat. Application Publ. No. 2003-0072964 (Thompson et al.),paragraphs 7-132; and FIGS. 1-8; U.S. Pat. Application Publ. No.2002-0182441 (Lamansky et al.),, paragraphs 13-165, including FIGS.1-9(g); U.S. Pat. No. 6,420,057 B1 (Ueda et al.), col. 1, line 57,through col. 88, line 17, including each compound I-1 through XXIV-12;U.S. Pat. No. 6,383,666 B1 (Kim et al.), col. 2, line 9, through col.21, lin3 67; U.S. Pat. Application Publ. No. 2001-0015432 A1 (Igarashiet al.), paragraphs 2-57, including compounds (1-1) through (1-30); U.S.Pat. Application Publ. No. 2001-0019782 A1 (Igarashi et al.), paragraphs13-126, including compounds (1-1) through (1 -70), and (2-1) through(2-20); U.S. Pat. Application Publ. No. 2002-0024293 (Igarashi et al.),paragraphs 7-95, including general formulas K-I through K-VI, andexample compounds (K-1) through (K-25); U.S. Pat. Application Publ. No.2002-0048689 A1 (Igarashi et al.), paragraphs 5-134, including compounds1-81, and example compounds (1-1) through (1-81); U.S. Pat. ApplicationPubl. No. 2002-0063516 (Tsuboyama et al.), paragraphs 31-161, includingeach compound 1-16; U.S. Pat. Application Publ. No. 2003-0068536(Tsuboyama et al.), paragraphs 31-168, including each compound in Tables1-17, corresponds to EP-1-239-526-A2; U.S. Pat. Application Publ. No.2003-0091862 (Tokito et al.), paragraphs 10-190, including each compoundin Tables 1-17, corresponds to EP-1-239-526-A2; U.S. Pat. ApplicationPubl. No. 2003-0096138 (Lecloux et al.), paragraphs 8-124, includingFIGS. 1-5; U.S. Pat. Application Publ. No. 2002-0190250 (Grushin etal.), paragraphs 9-191; U.S. Pat. Application Publ. No. 2002-0121638(Grushin et al.), paragraphs 8-125; U.S. Pat. Application Publ. No.2003-0068526 (Kamatani et al.), paragraphs 33-572, including eachcompound in Tables 1-23; U.S. Pat. Application Publ. No. 2003-0141809(Furugori et al.), paragraphs 29-207; U.S. Pat. Application Publ. No.2003-0162299 A1 (Hsieh et al.), paragraphs 8-42; WO 03/084972, (Stosselet al.), Examples 1-33; WO 02/02714 A2 ((Petrov et al.), pages 2-30,including each compound in Tables 1-5; EP 1-191-613 A1(Takiguchi etal.), paragraphs 26-87, including each compound in Tables 1-8,(corresponding to U.S. Pat. Application Publ. No. 2002-0064681); and EP1-191-614 A2 (Tsuboyama et al.), paragraphs 25-86, including eachcompound in Tables 1-7; which are incorporated herein by reference intheir entirety.

In one embodiment, the invention provides an organic light emittingdevice, comprising an anode, a cathode, and a phosphorescent emissiveregion disposed between the anode and the cathode, wherein the emissiveregion comprises an emissive material having the formula I:

wherein:

-   M is a second or third row transition metal; the dotted lines    represent optional double bonds;-   L is an ancillary ligand;-   X¹ is selected from C and N;-   X² is selected from C, N, O, S and P;-   X³ is selected from C and N; each Q is independently selected from a    chemical bond and —C(R′)₂—;-   Y¹ is C or N;-   Y² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N;-   R¹ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   a is 1 or 2;-   R³ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   c is 1 or 2;-   alternatively, R¹ and R³, taken together with Y¹ and Y², form a 5-    or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   Z¹ is C or N;-   Z² is C, N, O, S or P, wherein when Y¹ is N. Y² is C or N;-   R² is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   b is 1 or 2;-   R⁴ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;-   d is 1 or 2;-   alternatively, R² and R⁴, taken together with Z¹ and Z², form a 5-    or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   R⁵ and R⁶ are independently selected from H, alkyl, alkenyl,    alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl and heteroaryl; or    alternatively, R⁵ and R⁶, taken together with X¹, X² and X³ form a    5- or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   additionally or alternatively R¹ and R⁵ taken together with X¹, Q    and Y¹ form a 5- or 6-membered cyclic group or a 8- to 10-membered    fused bicyclic group, which may be optionally substituted with one    or more substituents independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   additionally or alternatively R² and R⁶ taken together with X³, Q    and Z¹ form a 5- or 6-membered cyclic group or a 8- to 10-membered    fused bicyclic group, which may be optionally substituted with one    or more substituents independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   each R¹ is independently selected from H, alkyl, alkenyl, alkynyl,    aralkyl, aryl and heteroaryl;-   n is 1 or 2; and-   m is 0 to 3, wherein when n is 1, m is 1 to 3, and when n is 2, m is    0;-   wherein at least one of X², Y² or Z² is C.

All value ranges, for example those given for n and m, are inclusiveover the entire range. Thus, for example, a range between 0-4 wouldinclude the values 0, 1, 2, 3 and 4.

In preferred embodiments of the invention, M is selected from Ru, Os,Re, Rh, Ir, Pd and Pt. In particularly preferred embodiments, M isselected from Os and Ru, and in still more preferred embodiments, M isOs. For certain preferred embodiments, Os is the preferred metal as itis relatively easy to oxidize. In further preferred embodiments, theemissive material has an oxidation potential that is more positive thanabout −0.7 volts, more preferably more positive than about −0.3 volts,and still more preferably more positive than about 0 volts, relative toFerrocene/Ferrocenium cation.

In preferred embodiments of the invention, X² of the compound accordingto the formula I is selected from C and N.

In another preferred embodiment, at least one of X², Y² or Z² is C andis selected to be a carbene donor.

In another preferred embodiment, the compound according to the formula Iis a neutral compound. Neutral compound may have the advantage of beingeasier to process in the manufacture of the device as they may bedeposited using sublimation techniques.

In the case where the compound according to the formula I is a chargedcompound, the compound will include a counterion to balance the charge.In this case the metal complex will have as positive charge ranging from1⁺ to 6⁺, and preferably from 1⁺ to 3⁺. The counterion may be selectedfrom any appropriate anion which does not interfere with the function ofthe compound in the device, for example, as an emissive material. Theanion is selected to be electrochemically inert over the operationalvoltage range of the device. Preferred counteranions are typicallyweakly coordinating anions. The term “weakly coordinating anion” is wellknown in the art per se and generally refers to a large bulky anioncapable of delocalization of the negative change of the anion. Suitableweakly coordinating anions, not all of which would be considered bulky,include, but are not limited to: PF₆ ⁻, BF₄ ⁻, SbCl₆ ⁻,trifluoromethansulfonate, BAr₄ ⁻(Ar=C₆F₅), BAr′₄⁻(Ar′=3,5-bis(trifluoromethyl)phenyl, and the like. The weaklycoordinating nature of such anions is known to those skilled in the artand described in the literature (S. Strauss et al., Chem. Rev., 1993,93, 927).

In a further preferred embodiment of the invention, R⁵ and R⁶ are takentogether with X¹, X² and X³ form a cyclic group (denoted as ring A) togive a compound having the formula II:

-   -   wherein:

-   M is a second or third row transition metal; the dotted lines    represent optional double bonds;

-   L is an ancillary ligand;

-   X¹ is selected from C and N;

-   X² is selected from C, N, O, S and P;

-   X³ is selected from C and N; ring A selected from a 5- or 6-membered    cyclic group or a 8- to 10-membered fused bicyclic group, wherein    ring A may be optionally substituted with one or more substituents    independently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,    N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or    SO₃R′ halo, aryl and heteroaryl;

-   each Q is independently selected from a chemical bond and —C(R′)₂—;

-   Y¹ is C or N;

-   Y² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N;

-   R¹ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;

-   a is 1 or 2;

-   R³ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;

-   c is 1 or 2;

-   alternatively, R¹ and R³, taken together with Y¹ and Y², form a 5-    or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;

-   Z¹ is C or N;

-   Z² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N;

-   R² is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;

-   b is 1 or 2;

-   R⁴ is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,    C(O)OR′, C(O)NR′₂, aryl and heteroaryl;

-   d is 1 or 2;

-   alternatively, R² and R⁴, taken together with Z¹ and Z², form a 5-    or 6-membered cyclic group or a 8- to 10-membered fused bicyclic    group, which may be optionally substituted with one or more    substituents independently selected from alkyl, alkenyl, alkynyl,    aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,    SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;

-   additionally or alternatively R¹ and R⁵ taken together with X¹, Q    and Y′ form a 5- or 6-membered cyclic group or a 8- to 10-membered    fused bicyclic group, which may be optionally substituted with one    or more substituents independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;

-   additionally or alternatively R² and R⁶ taken together with X³, Q    and Z¹ form a 5- or 6-membered cyclic group or a 8- to 10-membered    fused bicyclic group, which may be optionally substituted with one    or more substituents independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;

-   each R¹ is independently selected from H, alkyl, alkenyl, alkynyl,    aralkyl, aryl and heteroaryl;

-   n is 1 or 2;

-   m is 0 to 3, wherein when n is 1, m is 1 to 3, and when n is 2, m is    0; and wherein at least one of X², Y² or Z² is C.

In a further preferred embodiment of a compound according to the formulaII, the metal center is bound to two tridentate ligand (n=2, m=0) togive a compound having the formula III:

wherein R¹, R², R³, R⁴, X¹, X², X³, Y¹, Y², Z¹, Z², M, ring A, Q, a, b,c, and d are as described for a compound of the formula II.

Compounds of the present invention which comprise two tridentate ligandsare preferred. Such compounds are preferred as it is believed that suchligand configurations may improve the stability of the materials whenincorporated into an organic light emitting device. Further, suchmaterials may have the additional advantage of being more stable tosublimation during preferred deposition techniques such as OVPD.

In a further embodiment of the invention, ring A is selected to be aphenyl or pyridyl ring, which ring may be further substituted, to give acompound having the formula IV:

wherein

-   M is a second or third row transition metal;-   X² is selected from C or N;-   R⁷, R⁸ and R⁹ are independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; and    additionally or alternatively, R⁵ and R⁶ or R⁶ and R⁷ form a fused    5- or 6-membered cyclic group, wherein the fused cyclic group    optionally contains 1 to 3 ring heteroatoms and is optionally    substituted with one of more substituents selected from alkyl,    alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,    C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl;-   additionally or alternatively R¹ and R⁷ taken together with the    atoms to which they are attached form a 5- or 6-membered cyclic    group or a 8- to 10-membered fused bicyclic group, which may be    optionally substituted with one or more substituents independently    selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,    C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo,    aryl and heteroaryl;-   additionally or alternatively R² and R⁹ taken together with the    atoms to which they are attached form a 5- or 6-membered cyclic    group or a 8- to 10-membered fused bicyclic group, which may be    optionally substituted with one or more substituents independently    selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,    C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo,    aryl and heteroaryl;-   R¹, R², R³, R⁴′, Y², Z², M, Q, a, b, c, and d are as described for a    compound of the formula II.

In another embodiment of the invention, R¹ and R³ are selected to form aring B, and R² and R⁴ are selected to form a ring C to give a compoundhaving the formula V:

wherein:

-   ring B and ring C are independently selected from a 5- or 6-membered    cyclic group or a 8- to 10-membered fused bicyclic group, which may    be optionally substituted with one or more substituents    independently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,    N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or    SO₃R′ halo, aryl and heteroaryl, and X¹, X², X³, Y¹, Y², Z¹, Z², M,    L, Q, a, b, c, d, n and m are as described for a compound of the    formula II.

In a further embodiment of a compound according to the formula V, ring Ais selected to be a phenyl or pyridyl ring, which ring may be furthersubstituted, to give a compound having the formula VI:

wherein:

-   X² is selected from C or N;-   ring B and ring C are independently selected from a 5- or 6-membered    cyclic group or a 8- to 10-membered fused bicyclic group, which may    be optionally substituted with one or more substituents    independently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,    N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or    SO₃R′ halo, aryl and heteroaryl; R⁷, R⁸ and R⁹ are independently    selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,    C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo,    aryl and heteroaryl; and additionally or alternatively, R⁵ and R⁶ or    R⁶ and R⁷ form a fused 5- or 6-membered cyclic group, wherein the    fused cyclic group optionally contains 1 to 3 ring heteroatoms and    is optionally substituted with one of more substituents selected    from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl. Y¹, Y², Z¹, Z², M, Q, L, n, and m are as described for a    compound of the formula II.

In a preferred embodiment of a compound according to the formula VI, themetal center is bound to two tridentate ligand (n=2, m=0) to give acompound having the formula VII:

-   X² is selected from C or N;-   ring B and ring C are independently selected from a 5- or 6-membered    cyclic group or a 8- to 10-membered fused bicyclic group, which may    be optionally substituted with one or more substituents    independently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,    N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or    SO₃R′ halo, aryl and heteroaryl; R⁷, R⁸ and R⁹ are independently    selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,    C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo,    aryl and heteroaryl; and additionally or alternatively, R⁵ and R⁶ or    R⁶ and R⁷ form a fused 5- or 6-membered cyclic group, wherein the    fused cyclic group optionally contains 1 to 3 ring heteroatoms and    is optionally substituted with one of more substituents selected    from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl. Y¹, Y², Z¹, Z², M and Q are as described for a compound    of the formula II.

In preferred embodiments of the invention, Q is selected to be a linkinggroup such as —C(R′)₂—, rather than a chemical bond. A preferred linkinggroup is —CH₂—. For the tridentate ligands in formulas I-X, theinclusion of a linking group may allow the metal-binding sites of theligand to more closely achieve the preferred coordination geometry ofthe metal center. For example, in the preferred embodiments wherein M isOs, n=2 and m=0, the inclusion of linking groups for Q may allow the twotridentate ligands to more closely achieve an octahedral coordinationsphere around the osmium.

In a further embodiment of the invention, the emissive material has theformula VIII

wherein:

-   M is a second or third row transition metal;-   X² is selected from C or N;-   Q is a bond or —C(R′)₂—;-   R⁷, R⁸ and R⁹ are independently selected from alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; and    additionally or alternatively, R⁵ and R⁶ or R⁶ and R⁷ form a fused    5- or 6-membered cyclic group, wherein the fused cyclic group    optionally contains 1 to 3 ring heteroatoms and is optionally    substituted with one of more substituents selected from alkyl,    alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,    C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl;-   R¹² is selected from alkyl, aryl and aralkyl;-   R¹⁰ and R¹¹ are independently selected from H, alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;-   additionally or alternatively R¹⁰ and R¹¹, or R¹¹ and R¹² form a    fused 5- or 6-membered cyclic group, wherein the fused cyclic group    is optionally substituted with one of more substituents selected    from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl;-   R²² is selected from alkyl, aryl and aralkyl;-   R²⁰ and R²¹ are independently selected from H, alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; and-   additionally or alternatively R²⁰ and R²¹, or R²¹ and R²² form a    fused 5- or 6-membered cyclic group, wherein the fused cyclic group    is optionally substituted with one of more substituents selected    from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl; and each R′ is independently selected from H, alkyl,    alkenyl, alkynyl, aralkyl, aryl and heteroaryl.

In a further embodiment of the invention, the emissive material has theformula IX:

wherein

-   -   M is a second or third row transition metal;    -   X² is selected from C or N;    -   R⁷, R⁸ and R⁹ are independently selected from alkyl, alkenyl,        alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂,        CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; and        additionally or alternatively, R⁵ and R⁶ or R⁶ and R⁷ form a        fused 5- or 6-membered cyclic group, wherein the fused cyclic        group optionally contains 1 to 3 ring heteroatoms and is        optionally substituted with one of more substituents selected        from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,        C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′        halo, aryl and heteroaryl;

-   R¹² is selected from alkyl, aryl and aralkyl;

-   R¹⁰ and R¹¹ are independently selected from H, alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;

-   additionally or alternatively R¹⁰ and R¹¹, or R¹¹ and R¹² form a    fused 5- or 6-membered cyclic group, wherein the fused cyclic group    is optionally substituted with one of more substituents selected    from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl;

-   R²² is selected from alkyl, aryl and aralkyl;

-   R²⁰ and R²¹ are independently selected from H, alkyl, alkenyl,    alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN,    CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; and

-   additionally or alternatively R²⁰ and R²¹, or R²¹ and R²² form a    fused 5- or 6-membered cyclic group, wherein the fused cyclic group    is optionally substituted with one of more substituents selected    from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl; and each R′ is independently selected from H, alkyl,    alkenyl, alkynyl, aralkyl, aryl and heteroaryl.

In a further embodiment of the invention, the emissive material has theformula X:

wherein:

-   M is a second or third row transition metal;-   Y² and Z² are independently selected from C and N;-   R⁸, R⁹, R¹³, R¹⁴, R¹⁵, R¹⁶, R²³, R²⁴, R²⁵, and R²⁶ are independently    selected from H, alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂,    SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′    halo, aryl and heteroaryl;-   additionally or alternatively R⁸ and R⁹ form a fused 5- or    6-membered cyclic group, wherein the fused cyclic group is    optionally substituted with one of more substituents selected from    alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl; additionally or alternatively R¹³ and R⁴, or R¹⁴ and    R¹⁵, or R¹⁵ and R¹⁶ form a fused 5- or 6-membered cyclic group,    wherein the fused cyclic group is optionally substituted with one of    more substituents selected from alkyl, alkenyl, alkynyl, aralkyl,    O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂,    SOR′, or SO₃R′ halo, aryl and heteroaryl; and additionally or    alternatively R²³ and R²⁴, or R²⁴ and R²⁵, or R²⁵ and R²⁶ form a    fused 5- or 6-membered cyclic group, wherein the fused cyclic group    is optionally substituted with one of more substituents selected    from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,    C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and    heteroaryl; and each R′ is independently selected from H, alkyl,    alkenyl, alkynyl, aralkyl, aryl and heteroaryl.

Preferred substituents off of the aryl and heteroaryl groups includeCF₃, CN, CH₃, F, and phenyl. In preferred embodiments, the substituentR⁸ of compounds IV, VI, VII, VIII and IX is selected from CF₃, CN, CH₃,F, and phenyl, with CF₃ and CN being particularly preferred.

Preferred embodiments include osmium complexes which comprise twotridentate ligands each of which have a bond to the metal center from acarbenes and from an anionic phenyl ring, such as:

Other preferred embodiments include osmium complexes which comprise twotridentate ligands, each of which has two carbenes and one anionicphenyl ring, such as:

In another embodiment, the emissive material is a charged compound, asexemplified below:

When the emissive material comprises a charged compound, it isunderstood that the material will have associated with it a counterionto balance the charge.

In certain embodiments, the organic light emitting device may comprise amaterial, which may be an emissive material, wherein the materialcomprises two tridentate ligands bound to the metal center through atleast one carbon-metal bond, wherein carbon-metal bonds are notcarbene-metal bonds, as exemplified below:

The term “halo” or “halogen” as used herein includes fluorine, chlorine,bromine and iodine.

The term “alkyl” as used herein contemplates both straight and branchedchain alkyl radicals. Preferred alkyl groups are those containing fromone to fifteen carbon atoms and includes methyl, ethyl, propyl,isopropyl, butyl, isobutyl, tert-butyl, and the like. Additionally, thealkyl group may be optionally substituted with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR², cyclic-amino, NO₂, and OR,wherein each R is independently selected from H, alkyl, alkenyl,alkynyl, aralkyl, aryl and heteroaryl.

The term “cycloalkyl” as used herein contemplates cyclic alkyl radicals.Preferred cycloalkyl groups are those containing 3 to 7 carbon atoms andincludes cyclopropyl, cyclopentyl, cyclohexyl, and the like.Additionally, the cycloalkyl group may be optionally substituted withone or more substituents selected from halo, CN, CO₂R, C(O)R, NR²,cyclic-amino, NO₂, and OR.

The term “alkenyl” as used herein contemplates both straight andbranched chain alkene radicals. Preferred alkenyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkenyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “alkynyl” as used herein contemplates both straight andbranched chain alkyne radicals. Preferred alkyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkynyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR², cyclic-amino, NO₂, and OR.

The terms “aralkyl” as used herein contemplates an alkyl group that hasas a substituent an aromatic group. Additionally, the aralkyl group maybe optionally substituted on the aryl with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR², cyclic-amino, NO₂, and OR.

The term “heterocyclic group” as used herein contemplates non-aromaticcyclic radicals. Preferred heterocyclic groups are those containing 3 or7 ring atoms which includes at least one hetero atom, and includescyclic amines such as morpholino, piperdino, pyrrolidino, and the like,and cyclic ethers, such as tetrahydrofuran, tetrahydropyran, and thelike. Additionally, the heterocyclic group may be optionally substitutedwith one or more substituents selected from halo, CN, CO₂R, C(O)R, NR²,cyclic-amino, NO₂, and OR,

The term “aryl” or “aromatic group” as used herein contemplatessingle-ring groups and polycyclic ring systems. The polycyclic rings mayhave two or more rings in which two carbons are common by two adjoiningrings (the rings are “fused”) wherein at least one of the rings isaromatic, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl,heterocycles and/or heteroaryls. Additionally, the aryl group may beoptionally substituted with one or more substituents selected from halo,CN, CO₂R, C(O)R, NR², cyclic-amino, NO₂, and OR,

The term “heteroaryl” as used herein contemplates single-ringhetero-aromatic groups that may include from one to three heteroatoms,for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole,triazole, pyrazole, pyridine, pyrazine and pyrimidine, and the like. Theterm heteroaryl also includes polycyclic hetero-aromatic systems havingtwo or more rings in which two atoms are common to two adjoining rings(the rings are “fused”) wherein at least one of the rings is aheteroaryl, e.g., the other rings can be cycloalkyls, cycloalkenyls,aryl, heterocycles and/or heteroaryls. Additionally, the heteroarylgroup may be optionally substituted with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR², cyclic-amino, NO₂, and OR.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

Material Definitions:

As used herein, abbreviations refer to materials as follows: CBP:4,4′-N,N-dicarbazole-biphenyl m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine Alq₃: 8-tris-hydroxyquinolinealuminum Bphen: 4,7-diphenyl-1,10-phenanthroline n-BPhen: n-doped BPhen(doped with lithium) F₄-TCNQ: tetrafluoro-tetracyano-quinodimethanep-MTDATA: p-doped m-MTDATA (doped with F₄- TCNQ) Ir(ppy)₃:tris(2-phenylpyridine)-iridium Ir(ppz)₃: tris(1-phenylpyrazoloto,N,C(2′)iridium(III) BCP:2,9-dimethyl-4,7-diphenyl-1,10- phenanthroline TAZ:3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4- triazole CuPc: copperphthalocyanine. ITO: indium tin oxide NPD:N,N′-diphenyl-N-N′-di(1-naphthyl)- benzidine TPD:N,N′-diphenyl-N-N′-di(3-toly)-benzidine BA1q:aluminum(III)bis(2-methyl-8- hydroxyquinolinato)4-phenylphenolate mCP:1,3-N,N-dicarbazole-benzene DCM: 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran DMQA: N,N′-dimethylquinacridonePEDOT:PSS: an aqueous dispersion of poly(3,4- ethylenedioxythiophene)with polystyrenesulfonate (PSS)Experimental:

Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.Synthesis of Osmium Carbene Complex 1 (Os1)

A three neck 1000 mL round bottom flask was charged with 19.1 g ofbenzimidazole, 25.0 g of 1,3-diiodobenzene, 1.46 g of copper (I) iodide,44.7 g of potassium carbonate, 2.77 g of 1,10-phenathroline, and 500 mLof anhydrous N,N-dimethylformamide. The reaction mixture was heated toreflux under nitrogen for 2 days. After cooling to room temperature thereaction was filtered and the solvent removed from the filtrate byrotary evaporation. The crude product was purified by silica gel columnchromatography using 95% dichloromethane/methanol as the eluent. Thefractions containing the desired material were combined and the solventremoved by rotary evaporation. The product was crystallized from adichloromethane/ethyl acetate mixture. The product (18 g) was collectedby vacuum filtration as a white solid.

A 1000 mL round bottom flask was charged with 15 g of Compound A, 30 gof iodomethane, and 400 mL of N,N-dimethylformamide. The mixture washeated to approximately 60° C. for 18 hours. The mixture was filteredand the solids were washed with ethyl acetate. The solids were thenslurried in 400 ml of refluxing methanol, cooled, and then filtered toyield 20 g of the desired product as a white solid.

Step 3: OsH₄(PPh₃)₃

A 1000 mL three neck round bottom flask was charged with 3.6 g oftriphenylphosphine and 100 ml of ethanol. The mixture is heated toreflux and 1.0 g of ammonium hexachloroosmiate was added. A solution of0.43 g of sodium borohydride in 50 ml of ethanol is then added dropwise.The reaction is refluxed for 30 minutes and then cooled to roomtemperature. The off white solids are collected by vacuum filtration.The solids were washed successively with ethanol, water, ethanol, andhexane.

A 250 ml three neck round bottom flask was charged with 4.0 g ofcompound B, 3.3 g of OsH₄(PPh₃)₃, and 125 mL of N,N-dimethylformamide.The mixture was heated to reflux for 20 hours under nitrogen. Thesolvent was removed by rotary evaporation and the crude product waspurified by silica gel column chromatography using 80% hexanes/ethylacetate as the eluent. The fractions containing the desired product werecombined and the solvent removed by rotary evaporation. The product wascrystallized from an ethyl acetate/hexane mixture. The product wasconfirmed by mass spectroscopy and ¹H NMR.

The emission spectra for Os 1 as a solution in toluene (degassed) atroom temperature is shown in FIG. 3.

EXAMPLE 2

Synthesis osmium Carbene Complex OS2

To a Solution of CuBr₂ (26.8 g, 120 mmole) in anhydrous Acetontrile (500ml) at 0° C. was added t-butyl nitrite (21.1 ml, 160 mmole) dropwise,and then 3-amino-5-bromobenzotrifluoride (25 g, 104.1 mmole) was addeddropwise. The mixture was stirred at 0° C. for 1.5 h, then at roomtemperature for 16 h. The mixture was then concentrated to half of itsoriginal volume in vacuo, and then poured into 1 N HCl (620 ml). Thismixture was extracted with ether (400 ml). The organic layer was washedwith 1N HCl, dried (Na₂SO₄), filtered, concentrated in vacuo. Theresidue was purified by flash column chromatography on silica gel usinghexanes as the eluent and 18.33 g of OS 2-A was obtained. (57.8%)

A 1000 mL round bottom flask was charged with 13.33 g of compound OS2-A,12.43 g of benzimidazole, 1.668 g of copper (I) iodide, 3.15 g of1,10-phenanthroline, 59.93 g of cesium carbonate, and 500 ml ofN,N-dimethylformamide. The reaction mixture was heated to 150° C. undernitrogen for 60 hours. After cooling to room temperature; the reactionwas filtered and the solvent was removed from the filtrate by rotaryevaporation. The crude product was purified by silica gel columnchromatography using 95% dichloromethane/ethyl acetate mixture. Theproduct OS2-B (10 g, 60%) was collected by vacuum filtration as a whitesolid.

A 1000 ml round bottom flask was charged with 8 g of compound OS2—C, 60g of iodomethane and 500 ml of N,N-dimethylformamide. The mixture washeated to 41° C. for 70 hrs. 200 ml of toluene was added to induce theprecipitation. The mixture was filtered and the solids were washed withether to yield 9 g of OS2—C (64%).

A 50 ml round bottom flask was charged with 310 mg of compound OS2-C,230 mg of OsH₄(PPh₃)₃ and 35 ml of N,N-dimethylformamide. The mixturewas heated to 150° C. for 3 hrs under nitrogen. The reaction mixture wasdumped into water (150 ml) and extracted by ether. The ether was removedby rotary evaporation. The residue was purified by column chromatography(SiO₂, 80% hexanes/ethyl acetate) to yield OS2.

The emission spectra for Os2 as a solution in toluene (degassed) at roomtemperature is shown in FIG. 4.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. An organic light emitting device, comprising: an anode; a cathode;and a phosphorescent emissive region disposed between the anode and thecathode, wherein the emissive region comprises an emissive materialhaving the formula I:

wherein: M is a second or third row transition metal; the dotted linesrepresent optional double bonds; L is an ancillary ligand; X¹ isselected from C and N; X² is selected from C, N, O, S and P; X³ isselected from C and N; each Q is independently selected from a chemicalbond and —C(R′)₂—; Y¹ is C or N; Y² is C, N, O, S or P, wherein when Y¹is N, Y² is C or N; R¹ is selected from H, alkyl, alkenyl, alkynyl,aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl and heteroaryl; a is 1 or 2; R³is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′,C(O)NR′₂, aryl and heteroaryl; c is 1 or 2; alternatively, R¹ and R³,taken together with Y¹ and Y², form a 5- or 6-membered cyclic group or a8- to 10-membered fused bicyclic group, which may be optionallysubstituted with one or more substituents independently selected fromalkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;Z¹ is C or N; Z² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N;R² is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,C(O)OR′, C(O)NR′₂, aryl and heteroaryl; b is 1 or 2; R⁴ is selected fromH, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl andheteroaryl; d is 1 or 2; alternatively, R² and R⁴, taken together withZ¹ and Z², form a 5- or 6-membered cyclic group or a 8- to 10-memberedfused bicyclic group, which may be optionally substituted with one ormore substituents independently selected from alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; R⁵ and R⁶ areindependently selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,C(O)OR′, C(O)NR′₂, aryl and heteroaryl; or alternatively, R⁵ and R⁶,taken together with X¹, X² and X³ form a 5- or 6-membered cyclic groupor a 8- to 10-membered fused bicyclic group, which may be optionallysubstituted with one or more substituents independently selected fromalkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;additionally or alternatively R¹ and R⁵ taken together with X¹, Q and Y¹form a 5- or 6-membered cyclic group or a 8- to 10-membered fusedbicyclic group, which may be optionally substituted with one or moresubstituents independently selected from alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; additionally oralternatively R² and R⁶ taken together with X³, Q and Z′ form a 5- or6-membered cyclic group or a 8- to 10-membered fused bicyclic group,which may be optionally substituted with one or more substituentsindependently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; each R′ is independently selected fromH, alkyl, alkenyl, alkynyl, aralkyl, aryl and heteroaryl; n is 1 or 2;and m is 0 to 3, wherein when n is 1, m is 1 to 3, and when n is 2, m is0; wherein at least one of X², Y² or Z² is C and is selected to be acarbene donor.
 2. The device of claim 1, wherein M is selected from Ru,Os, Re, Rh, Ir, Pd and Pt.
 3. The device of claim 2, wherein M isselected from Os and Ru.
 4. The device of claim 3, wherein M is Os. 5.The device of claim 1, wherein the emissive material comprises a neutralcompound.
 6. The device of claim 1, wherein the emissive materialcomprises a charged compound.
 7. The device of claim 1, wherein theemissive material has the formula II:

wherein: M is a second or third row transition metal; the dotted linesrepresent optional double bonds; L is an ancillary ligand; X¹ isselected from C and N; X² is selected from C, N, O, S and P; X³ isselected from C and N; ring A selected from a 5- or 6-membered cyclicgroup or a 8- to 10-membered fused bicyclic group, wherein ring A may beoptionally substituted with one or more substituents independentlyselected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryland heteroaryl; each Q is independently selected from a chemical bondand —C(R′)₂—; Y¹ is C or N; Y² is C, N, O, S or P, wherein when Y¹ is N,Y² is C or N; R³ is selected from H, alkyl, alkenyl, alkynyl, aralkyl,C(O)R′, C(O)OR′, C(O)NR′₂, aryl and heteroaryl; a is 1 or 2; R³ isselected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′,C(O)NR′₂, aryl and heteroaryl; c is 1 or 2; alternatively, R¹ and R³,taken together with Y¹ and Y², form a 5- or 6-membered cyclic group or a8- to 10-membered fused bicyclic group, which may be optionallysubstituted with one or more substituents independently selected fromalkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;Z¹ is C or N; Z² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N;R² is selected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′,C(O)OR′, C(O)NR′₂, aryl and heteroaryl; bis 1 or 2; R⁴ is selected fromH, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl andheteroaryl; d is 1 or 2; alternatively, R² and R⁴, taken together withZ¹ and Z², form a 5- or 6-membered cyclic group or a 8- to 10-memberedfused bicyclic group, which may be optionally substituted with one ormore substituents independently selected from alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; each R′ is independentlyselected from H, alkyl, alkenyl, alkynyl, aralkyl, aryl and heteroaryl;n is 1 or 2; m is 0 to 3, wherein when n is 1, m is 1 to 3, and when nis 2, m is 0; and wherein at least one of X², Y² or Z² is C and isselected to be a carbene donor.
 8. The device of claim 7, wherein M isselected from Ru, Os, Re, Rh, Ir, Pd and Pt.
 9. The device of claim 8,wherein M is selected from Os and Ru.
 10. The device of claim 9, whereinM is Os.
 11. The device of claim 7, wherein the emissive material hasthe formula III:

wherein M is a second or third row transition metal; the dotted linesrepresent optional double bonds; X¹ is selected from C and N; X² isselected from C, N, O, S and P; X³ is selected from C and N; ring Aselected from a 5- or 6-membered cyclic group or a 8- to 10-memberedfused bicyclic group, wherein ring A may be optionally substituted withone or more substituents independently selected from alkyl, alkenyl,alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃,NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; each Q isindependently selected from a chemical bond and —C(R′)₂—; Y¹ is C or N;Y² is C, N, O, S or P, wherein when Y¹ is N, Y² is C or N; R¹ isselected from H, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′,C(O)NR′₂, aryl and heteroaryl; a is 1 or 2; R³ is selected from H,alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl andheteroaryl; c is 1 or 2; alternatively, R¹ and R³, taken together withY¹ and Y², form a 5- or 6-membered cyclic group or a 8- to 10-memberedfused bicyclic group, which may be optionally substituted with one ormore substituents independently selected from alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; Z¹ is C or N; Z² is C, N,O, S or P, wherein when Y¹ is N, Y² is C or N; R² is selected from H,alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl andheteroaryl; b is 1 or 2; R⁴ is selected from H, alkyl, alkenyl, alkynyl,aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl and heteroaryl; d is 1 or 2;alternatively, R² and R⁴, taken together with Z¹ and Z², form a 5- or6-membered cyclic group or a 8- to 10-membered fused bicyclic group,which may be optionally substituted with one or more substituentsindependently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; additionally or alternatively R¹ and R⁵taken together with X¹, Q and Y¹ form a 5- or 6-membered cyclic group ora 8- to 10-membered fused bicyclic group, which may be optionallysubstituted with one or more substituents independently selected fromalkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;additionally or alternatively R² and R⁶ taken together with X³, Q and Z¹form a 5- or 6-membered cyclic group or a 8- to 10-membered fusedbicyclic group, which may be optionally substituted with one or moresubstituents independently selected from alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; each R′ is independentlyselected from H, alkyl, alkenyl, alkynyl, aralkyl, aryl and heteroaryl;and wherein at least one of X², Y² or Z² is C and is selected to be acarbene donor.
 12. The device of claim 11, wherein M is selected fromRu, Os, Re, Rh, Ir, Pd and Pt.
 13. The device of claim 12, wherein M isselected from Os and Ru.
 14. The device of claim 13, wherein M is Os.15. The device of claim 1, wherein the emissive material has the formulaIV:

wherein M is a second or third row transition metal; the dotted linesrepresent optional double bonds; X¹ is selected from C and N; X² isselected from C and N; X³ is selected from C and N; Y¹ is C or N; Y² isC, N, O, S or P, wherein when Y¹ is N, Y² is C or N; R¹ is selected fromH, alkyl, alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl andheteroaryl; a is 1 or 2; R³ is selected from H, alkyl, alkenyl, alkynyl,aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl and heteroaryl; cis 1 or 2;alternatively, R¹ and R³, taken together with Y¹ and Y², form a 5- or6-membered cyclic group or a 8- to 10-membered fused bicyclic group,which may be optionally substituted with one or more substituentsindependently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; Z¹ is C or N; Z² is C, N, O, S or P,wherein when Y¹ is N, Y² is C or N; R is selected from H, alkyl,alkenyl, alkynyl, aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl andheteroaryl; b is 1 or 2; R⁴ is selected from H, alkyl, alkenyl, alkynyl,aralkyl, C(O)R′, C(O)OR′, C(O)NR′₂, aryl and heteroaryl; d is 1 or 2;alternatively, R² and R⁴, taken together with Z¹ and Z², form a 5- or6-membered cyclic group or a 8- to 10-membered fused bicyclic group,which may be optionally substituted with one or more substituentsindependently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; R⁷, R⁸ and R⁹ are independentlyselected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryland heteroaryl; and additionally or alternatively, R⁵ and R⁶ or R⁶ andR⁷ form a fused 5- or 6-membered cyclic group, wherein the fused cyclicgroup optionally contains 1 to 3 ring heteroatoms and is optionallysubstituted with one of more substituents selected from alkyl, alkenyl,alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃,NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; additionally oralternatively R¹ and R⁷ taken together with the atoms to which they areattached form a 5- or 6-membered cyclic group or a 8- to 10-memberedfused bicyclic group, which may be optionally substituted with one ormore substituents independently selected from alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; additionally oralternatively R² and R⁹ taken together with the atoms to which they areattached form a 5- or 6-membered cyclic group or a 8- to 10-memberedfused bicyclic group, which may be optionally substituted with one ormore substituents independently selected from alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; each R′ is independentlyselected from H, alkyl, alkenyl, alkynyl, aralkyl, aryl and heteroaryl;and wherein at least one of X², Y² or Z² is C and is selected to be acarbene donor.
 16. The device of claim 15, wherein M is selected fromRu, Os, Re, Rh, Ir, Pd and Pt.
 17. The device of claim 16, wherein M isselected from Os and Ru.
 18. The device of claim 17, wherein M is Os.19. The device of claim 1, wherein the emissive material has the formulaV:

wherein: M is a second or third row transition metal; the dotted linesrepresent optional double bonds; L is an ancillary ligand; X¹ isselected from C and N; X² is selected from C and N; X³ is selected fromC and N; ring A selected from a 5- or 6-membered cyclic group or a 8- to10-membered fused bicyclic group, wherein ring A may be optionallysubstituted with one or more substituents independently selected fromalkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;ring B and ring C are independently selected from a 5- or 6-memberedcyclic group or a 8- to 10-membered fused bicyclic group, which may beoptionally substituted with one or more substituents independentlyselected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryland heteroaryl; each Q is independently selected from a chemical bondand —C(R′)₂—; Y¹ is C or N; Y² is C, N, O, S or P, wherein when Y¹ is N,Y² is C or N; Z¹ is C or N; Z² is C, N, O, S or P, wherein when Y¹ is N.Y² is C or N; each R¹ is independently selected from H, alkyl, alkenyl,alkynyl, aralkyl, aryl and heteroaryl; n is 1 or 2; m is 0 to 3, whereinwhen n is 1, m is 1 to 3, and when n is 2, m is 0; and wherein at leastone of X², Y² or Z² is C and is selected to be a carbene donor.
 20. Thedevice of claim 19, wherein M is selected from Ru, Os, Re, Rh, Ir, Pdand Pt.
 21. The device of claim 20, wherein M is selected from Os andRu.
 22. The device of claim 21, wherein M is Os.
 23. The device of claim1, wherein the emissive material has the formula VI:

wherein: M is a second or third row transition metal; the dotted linesrepresent optional double bonds; L is an ancillary ligand; X¹ isselected from C and N; X² is selected from C and N; X³ is selected fromC and N; ring B and ring C are independently selected from a 5- or6-membered cyclic group or a 8- to 10-membered fused bicyclic group,which may be optionally substituted with one or more substituentsindependently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; each Q is independently selected from achemical bond and —C(R′)₂—; Y¹ is C or N; Y² is C, N, O, S or P, whereinwhen Y¹ is N, Y² is C or N; Z¹ is C or N; Z² is C, N, O, S or P, whereinwhen Y¹ is N, Y² is C or N; R⁷, R⁸ and R⁹ are independently selectedfrom alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl andheteroaryl; and additionally or alternatively, R⁵ and R⁶ or R⁶ and R⁷form a fused 5- or 6-membered cyclic group, wherein the fused cyclicgroup optionally contains 1 to 3 ring heteroatoms and is optionallysubstituted with one of more substituents selected from alkyl, alkenyl,alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃,NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; each R′ isindependently selected from H, alkyl, alkenyl, alkynyl, aralkyl, aryland heteroaryl; n is 1 or 2; m is 0 to 3, wherein when n is 1, m is 1 to3, and when n is 2, mis 0; and wherein at least one of X², Y² or Z² is Cand is selected to be a carbene donor.
 24. The device of claim 23,wherein M is selected from Ru, Os, Re, Rh, Ir, Pd and Pt.
 25. The deviceof claim 24, wherein M is selected from Os and Ru.
 26. The device ofclaim 25, wherein M is Os.
 27. The device of claim 1, wherein theemissive material has the formula VII:

wherein: M is a second or third row transition metal; the dotted linesrepresent optional double bonds; L is an ancillary ligand; X¹ isselected from C and N; X² is selected from C and N; X³ is selected fromC and N; ring B and ring C are independently selected from a 5- or6-membered cyclic group or a 8- to 10-membered fused bicyclic group,which may be optionally substituted with one or more substituentsindependently selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; each Q is independently selected from achemical bond and —C(R′)₂—; Y¹ is C or N; Y² is C, N, O, S or P, whereinwhen Y¹ is N, Y² is C or N; Z¹ is C or N; Z² is C, N, O, S or P, whereinwhen Y¹ is N, Y² is C or N; R⁷, R⁸ and R⁹ are independently selectedfrom alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′,C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl andheteroaryl; and additionally or alternatively, R⁵ and R or R⁶ and R⁷form a fused 5- or 6-membered cyclic group, wherein the fused cyclicgroup optionally contains 1 to 3 ring heteroatoms and is optionallysubstituted with one of more substituents selected from alkyl, alkenyl,alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃,NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; each R′ isindependently selected from H, alkyl, alkenyl, alkynyl, aralkyl, aryland heteroaryl; and wherein at least one of X², Y² or Z² is C and isselected to be a carbene donor.
 28. The device of claim 27, wherein M isselected from Ru, Os, Re, Rh, Ir, Pd and Pt.
 29. The device of claim 28,wherein M is selected from Os and Ru.
 30. The device of claim 29,wherein M is Os.
 31. The device of claim 1, wherein the emissivematerial has the formula VIII

wherein: M is a second or third row transition metal; X² is selectedfrom C or N; Q is a bond or —C(R′)₂—; R⁷, R⁸ and R⁹ are independentlyselected from alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′,C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryland heteroaryl; and additionally or alternatively, R⁵ and R⁶ or R⁶ andR⁷ form a fused 5- or 6-membered cyclic group, wherein the fused cyclicgroup optionally contains 1 to 3 ring heteroatoms and is optionallysubstituted with one of more substituents selected from alkyl, alkenyl,alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃,NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; R¹² is selected fromalkyl, aryl and aralkyl; R¹⁰ and R¹¹ are independently selected from H,alkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;additionally or alternatively R¹⁰ and R¹¹, or R¹¹ and R¹² form a fused5- or 6-membered cyclic group, wherein the fused cyclic group isoptionally substituted with one of more substituents selected fromalkyl, alkenyl, alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′,C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl;R²² is selected from alkyl, aryl and aralkyl; R²⁰ and R²¹ areindependently selected from H, alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; and additionally or alternatively R²⁰and R²¹, or R²¹ and R²² form a fused 5- or 6-membered cyclic group,wherein the fused cyclic group is optionally substituted with one ofmore substituents selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; and each R′ is independently selectedfrom H, alkyl, alkenyl, alkynyl, aralkyl, aryl and heteroaryl.
 32. Thedevice of claim 31, wherein M is selected from Ru, Os, Re, Rh, Ir, Pdand Pt.
 33. The device of claim 32, wherein M is selected from Os andRu.
 34. The device of claim 33, wherein M is Os.
 35. The device of claim1, wherein the emissive material has the formula X:

wherein: M is a second or third row transition metal; Y² and Z² areindependently selected from C and N; R⁸, R⁹, R¹³, R¹⁴, R¹⁵, R¹⁶, R²⁵,and R²⁶ are independently selected from H. alkyl, alkenyl, alkynyl,aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂,SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; additionally oralternatively R⁸ and R⁹ form a fused 5- or 6-membered cyclic group,wherein the fused cyclic group is optionally substituted with one ofmore substituents selected from alkyl, alkenyl, alkynyl, aralkyl, O—R′,N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′, orSO₃R′ halo, aryl and heteroaryl; additionally or alternatively R¹³ andR⁴, or R¹⁴ and R⁵, or R¹⁵ and R¹⁶ form a fused 5- or 6-membered cyclicgroup, wherein the fused cyclic group is optionally substituted with oneof more substituents selected from alkyl, alkenyl, alkynyl, aralkyl,O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃, NO₂, SO₂, SOR′,or SO₃R′ halo, aryl and heteroaryl; and additionally or alternativelyR²³ and R²⁴, or R²⁴ and R²⁵, or R²⁵ and R²⁶ form a fused 5- or6-membered cyclic group, wherein the fused cyclic group is optionallysubstituted with one of more substituents selected from alkyl, alkenyl,alkynyl, aralkyl, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂, CN, CF₃,NO₂, SO₂, SOR′, or SO₃R′ halo, aryl and heteroaryl; and each R′ isindependently selected from H, alkyl, alkenyl, alkynyl, aralkyl, aryland heteroaryl.
 36. The device of claim 35, wherein M is selected fromRu, Os, Re, Rh, Ir, Pd and Pt.
 37. The device of claim 36, wherein M isselected from Os and Ru.
 38. The device of claim 37, wherein M is Os.